Detecting DJGPP, Cygwin, Mingwin and EMX OS/2

[Akim Demaille]

Currently to find out if we are under one of these environments
configure runs a compilation, typically to see if __CYGWIN__ is
defined etc.

Can't we do this kind of checks *without* compiling?  IIUC correctly,
for Autoconf to be really DJGPP aware, we'd need to change things
which do not depend upon compilation, so it is somewhat unfortunate.

Anybody would have solutions to provide?

        Akim